It is well known from color-matching experiments that the human visual system, under normal conditions, is trichromatic in nature. Color information is acquired in the retina by absorption of light in three distinct types of sensor cells (the "cones") having different broad-band spectral sensitivities. The neural signals generated in the retina, therefore, can be represented mathematically as a vector in a 3-dimensional space. A number of different 3-dimensional vector spaces have been used to represent visual colors; most notable are those defined over the years by the Commission Internationale de l'Eclairage (CIE), such as CIE 1931 XYZ, CIE 1976 L*a*b* (CIELAB), and CIE 1976 L*u*v* (CIELUV). In these spaces, a point represents a color visually distinct from those represented by all other points. The CIE has established and recommended practices for measuring and computing the coordinates in these spaces corresponding to real objects and illuminants; these practices together are known as colorimetry.
A corollary of the trichromatic nature of color vision is that the satisfactory reproduction of colors requires the use of 3 independent colorants. Three degrees of freedom are required in the color-reproduction system in order to vary the color stimulus over the 3 dimensions sensed by the human retina. For instance, color television and computer monitors use an additive set of red, green, and blue (RGB) phosphors. Color photography is based on a subtractive set of cyan, magenta and yellow (CMY) dyes. A number of printers and copiers also rely on overprints of CMY colorants.
Nonetheless, many systems use more than the required 3 colorants. Any reproduction process has a limited gamut, or range, of reproducible colors. The use of additional colorants can expand the gamut. Furthermore, particular additional colorants may be used to stabilize the reproduction process for particularly important colors, such as neutrals.
Most commonly, black is added to the set of subtractive primaries. In offset printing, for example, the use of black ink increases the dynamic range of the process, by permitting the reproduction of darker neutrals. It also stabilizes the process somewhat against mis-registration of the halftone screens and batch variations in the primary inks.
Examples of such processes are:
offset lithography, in which the "four-color process" employs printing inks as colorants applied to paper, in a halftone screen pattern, by a printing press; PA1 gravure printing, which also employs printing inks applied to paper; PA1 off-press proofing systems, which employ toners as colorants to simulate the effect of an offset press; PA1 computer-driven printers, which use a variety of technologies and colorants, such as jettable inks, toners, and dyes, applied in various continuous-tone, halftone, or dithered patterns.
Most of these processes make use of three chromatic colorants. In addition they may use an achromatic, or black, colorant (abbreviated as K). Thus, the chief application of the current invention is for CMYK systems, although it is clearly generalizable to systems using additional chromatic colorants, such as red, orange, pink, green, or blue.
Since the colorant space in these CMYK systems has a higher dimension (4) than the visual color spaces (3), there is generally no unique way to reproduce a given color. Typically, in fact, there is an infinite number of possible CMYK overprints that can render the same visual color. The resulting ambiguity in determining the rendering of colors has to be resolved by imposing constraints and aims on the overprints. For offset printing, for example, these take the form of restrictions on the total area coverage (TAC) of the 4 halftone separations and preferences in the utilization of black ink; these restrictions and preferences arise mostly from practical considerations of press control, quality assurance, cost reduction, etc., rather than from the intrinsic nature of color reproduction.
The current invention is a method of imposing such constraints and aims in a flexible and useful manner.
Color separations for offset printing have traditionally been produced from photographic originals through the use of RGB separation filters. Originally, these methods involved optical exposure of photographic transparencies. More recently, electronic scanning devices (first analog and subsequently digital) have been employed to separate the red, green, and blue content of the original image and convert the information into electrical signals. Typically, these RGB signals are then converted to CMY signals, which are then used to drive the printer, the film writer, or other output device. The simplest form of this conversion is the complement function: EQU C=1-R EQU M=1-G EQU Y=1-B
(Here the signals are assumed to range over the interval 0, 1!.) This simple form is not usually satisfactory, because the physics of the CMY color-reproduction process is complex and not directly related to the spectral sensitivities of the RGB filters. An empirical color-correction matrix is usually needed to convert RGB to CMY in such a way as to preserve acceptable fidelity in the reproduced colors.
Subsequently, a K signal is generated, in order to add black to the CMY overprints. There are many ways to do this, and scanning systems provide many options that can be selected by the operator. Frequently, for instance, it is desirable to avoid using black entirely in the lighter parts of the image, but to increase its use progressively in the darker parts. In some cases, there is a preference to restrict the use of black to neutral colors (grays) and near-neutrals, while, in others, black continues to be used even for moderately saturated (but dark) colors In general, these possibilities can be implemented by making K a function of the primary signals. In particular, a common approach is to make K a function of the minimum of C, M, and Y (for instance, Gaulke and Jung, U.S. Pat. No. 4,482,917).
When black is added to an overprint, the amount of primary colorant must be reduced, in order to keep the color from darkening. Certain CMY overprints reproduce shades of gray and are, therefore, visually equivalent to certain quantities of black colorant printed alone. These neutral overprints typically require an approximately equal balance of the cyan, magenta, and yellow colorants, although the balance is not exact for most processes. These equivalent overprints can be used to compute the reduction in CMY that should accompany the addition of K in order to preserve an approximate shade of gray in a neutral overprint. If black is used in the printing of chromatic colors, somewhat different methods may be required to preserve color fidelity (Gaulke and Jung, op. cit.).
Various strategies have developed in the printing industry for using more or less black in printing various colors and adjusting the amount of primary colorant accordingly. These have been known as Under Color Removal (UCR), Grey Component Replacement (GCR), and Under Color Addition (UCA). Unfortunately, these terms have been used differently by different writers and have lost the precise meaning they may once have had. Accordingly, here these terms will be avoided, and a distinction will be made instead between the use of black on-axis (i.e., for neutrals) and off-axis (i.e., for chromatic colors). Regardless of the terminology, however, the usual approach has been to convert RGB to an initial CMY signal; K is generated on the basis of this CMY; the CMY is then readjusted, on the basis of K, in order to approximate the color that would have been printed by the initial CMY. Thus, the signal-processing chain is RGB.fwdarw.(CMY).sub.i .fwdarw.CMYK!, where the subscript "i" distinguishes the initial values.
Most of these earlier methods cannot automatically preserve color fidelity to a high degree of accuracy, because the conversion of RGB to (CMY).sub.i and the conversion of (CMY).sub.i to CMYK are not based on realistic models of the color-reproduction process. Scanning devices for the graphic arts, therefore, provide (analog or digital) controls so that a trained operator can adjust these conversions empirically to improve the results. This is a difficult procedure, requiring considerable time and skill. More recently, there have appeared a number of digital systems, based on colorimetry, that represent an improved technology. In these systems, a conversion is first made from the RGB scanner signals to a device-independent color space, such as CIELAB. This conversion, e.g., RGB.fwdarw.L*a*b*!, is based on a calorimetric characterization of the scanner. This is followed by a conversion from the device-independent space to the CMYK signals that drive the output device. This second conversion is based on a colorimetric characterization of the reproduction process (offset printing, off-press proofing, inkjet printing, dye-sublimation printing, etc.), so that the colors that are printed will be visual matches to the colors that were scanned, since they correspond to the same CIELAB coordinates.
The mathematics of this second conversion involves the inversion of a model of the process. Color patches are printed with various combinations of the CMYK control signals; calorimetric measurements and computations then yield the corresponding visual colors, represented, for instance, in CIELAB. A mathematical model can then be fitted to the colorimetric data, resulting in a representation of the CMYK.fwdarw.L*a*b*! transformation. The inverse of this transformation must then be computed in order to implement the required conversion L*a*b*.fwdarw.CMYK!. This computation is generally implemented by an iterative search technique, conducted at a regular sampling of points in CIELAB space. The results of the inversion are tabulated, and various interpolation methods can subsequently be used to compute an approximation to the inverse at arbitrary points.
The same problem that occurred in the older technology now reappears in a new form. Just as there is no unique transformation from RGB to CMYK, there is no unique inverse to CMYK.fwdarw.L*a*b*! of the form L*a*b*.fwdarw.CMYK!. In order to remove the ambiguity in the inverse, constraints must be applied.
A typical approach to this problem is disclosed in Van de Capelle et al. (U.S. Pat. No. 5,402,253), which is incorporated herein by reference. As in the older technology, the CMY signals are regarded as the independent dimensions of the colorant space; K, on the other hand, is treated as an extra dimension that must be assigned a specific value by imposing an additional constraint or relation. For instance, K could be assigned a constant value. This would obviously remove the ambiguity in the transformation and permit a unique inverse; however, it would not be of much practical use, since, if K were fixed to a constant non-zero value, it would be impossible to render white by leaving areas of an image blank. Therefore, the referenced patent teaches a more general approach in which K is made to depend on C, M, and Y, and, in particular, may depend on the minimum of C, M, and Y. This is similar to the approach used in the older technology; there is an important difference, however, in that the dependence is not on an initial (CMY).sub.i which is subsequently readjusted. Instead, the relation that is imposed is a dependence of K on the final CMY. This is possible, in the newer technology, because the relation becomes a condition imposed on the iterative search for an inverse, rather than a particular step in a processing sequence.
Another way to look at this is that the relation between K and CMY defines a particular 3-dimensional subspace of CMYK-space. This subspace is selected in such a way that each overprint in the subspace reproduces a different color; i.e., no two points in the subspace map to the same point in CIELAB space. Thus, within the subspace, the inverse is unique.
A similar approach is disclosed in Rolleston et al. (U.S. Pat. No. 5,305,119 and No. 5,528,386). These authors similarly impose a relation between K and CMY. However, their method establishes the form of this relation prior to the calorimetric characterization. The color patches that are printed and colorimetrically analyzed consist of a sampling over CMY-space with the inclusion of K in the overprints as required by the relation. The model is then defined over this 3-dimensional space, and the 4-dimensional ambiguity is avoided from the beginning. The method is less flexible than that of Van de Capelle et al., however, since each choice of the relation requires that a different set of patches be printed and analyzed. For certain applications, such as graphic arts, it is more convenient to define and fit a calorimetric model over the 4-dimensional CMYK-space; various 3-dimensional subspaces can then be selected subsequently, at will, to obtain various behaviors of K with respect to CMY.
The technology described above is deficient in its ability to handle certain practical issues and requirements. For instance, many printing processes have a limitation in the total amount of colorant that can be deposited on a substrate without smearing or running. In offset printing, this is a limit on TAC (total area coverage), which is the sum of the fractional areas covered by the halftone dots in the 4 separations; it can be computed simply as (C+M+Y+K). The TAC limit is generally less than 4.0 ("400% dot coverage") and often less than 3.0 ("300% dot coverage") in typical offset presses. These constraints are difficult to impose within the cited prior art.
Another deficiency in the art is in providing separate controls for the on-axis and off-axis usage of black colorant. The relation between K and CMY, if only a function of a single variable (the minimum of C, M, and Y), may provide a desired on-axis usage, but may then fail to provide the off-axis behavior that some printers may require.
Another deficiency has to do with alternative interpretations of "neutral". In a colorimetric sense, "neutral", or "achromatic", may be defined as "having the same chromaticity as the illuminant". Since the paper stock may impart a slight cast to the chromaticity at the white point, it may be more practical to define "neutral" as "having the same chromaticity as that of the light of the illuminant after reflection from the substrate". This may be regarded as the "true neutral", because the entire gray scale, from white to black, can then be reproduced at the same chromaticity. On the other hand, it has been common practice in offset printing to define "neutral" conventionally, in terms of certain proportions of the printing inks--typically, equal amounts of magenta and yellow, overprinted with a somewhat greater amount of cyan. These overprints, combined with various amounts of black, may not exactly reproduce a true neutral, but they are a close approximation and are preferred by many press operators for practical considerations of press control. Still another possibility, more applicable to computer-driven printers than to printing presses, is to define "neutral" in terms of equal, or balanced, amounts of the 3 primaries; this choice allows the primaries to range independently up to their maximum value, thus maximizing the gamut and dynamic range of the process, even though the darkest "balanced neutral" may have an appreciable color cast relative to the substrate chromaticity. It is difficult to accommodate these possible alternatives within the framework of the existing technology.